Dramatic living arrangements between two completely different species are not confined to anemones and their clownfish lodgers.The relationship that sometimes occurs between shrimps and gobies is equally fascinating.

How does the shrimp and goby relationship world?

Some species of shrimps and gobies have a living arrangement that benefits both. The animals cohabit in shared burrows, excavated from the sands of reef lagoons or among coral heads in sheltered areas. The shrimp is extremely active, digging out the burrow and keeping it clear of debris, while the goby stands guard at the burrow's entrance, looking out for approaching danger in the form of predators.

What does each partner gain from the relationship?

The relationship seems to work because each of the partners has an Achilles heel that is compensated for by the other. The weakness of the shrimp is its poor eyesight, which makes it a potentially easy meal for predators. On the other hand, the goby has excellent eyesight and is able to keep a lookout for danger on the shrimp's behalf, but is far less capable than its crustacean housemate of excavating its own burrow. It benefits from the protection that the shrimp's burrow provides. Both partners survive for longer with these living arrangements than they would alone.

How does the goby warn the shrimp of impending danger?

If an inquisitive predator gets too close for comfort, the goby will respond by reversing into the burrow refuge. As it does so, it also communicates the danger to the shrimp by wiggling its tail as a warning.The shrimp picks up the warning through its antennae -- whenever it leaves the burrow it keeps one of its feelers in constant contact with the fish, ready to pick up these warning messages and prepared at a moment's notice to take heed and duck into the hole.

Do gobies and shrimps have to live together?

Several species of shrimps and gobies live alongside one another in this way. Some are described as "obligate" burrow sharers, meaning that they cannot live outside this symbiotic partnership; others are "facultative" burrow sharers and can live independently, although the benefits of the arrangement mean that often they do live in alliance with their shrimp or fish partner. However studies have shown that shrimps gain a far greater benefit from living with an obligate symbiont than with a facultative one.

For example, when they live with the orangespotted goby (Nes iongus), an obligate partner, they can spend three times as long outside their burrow -- meaning more digging and foraging time -- than when they live with a facultative partner such as the notchtongue goby (Bathygobius curacoq). This fish does not provide the shrimp with any special warning signals, other than a head-first dash into the burrow. Even so, the shrimps are able to spend longer outside the burrow when in association with a fish partner than they could alone.

How do gobies and shrimps find each other?

Long flat plains of sandy sea bed are extremely hazardous for small animals such as shrimps and gobies, so they need to find a partner quickly. Gobies use their eyesight to look out for shrimps, while the shrimps seek out partners by attuning to the fish's smell. Each is strongly attracted to the other, helping them locate and benefit each other with the mutual protection that their partnership provides.